1. Field of the Invention
This invention relates to a method for cladding a corrosion resistant high alloy material on surfaces of crossed cavities which are formed internally in metal vessels to be used as component parts of petroleum and natural gas boring equipment, including valves, well head parts and the like.
2. Description of the Prior Art
Metal vessels include, for example, gate valves which have been widely used due to their high pressure sealability and easier maintenance. In this connection, there have been proposed various methods for the improvement of durability of valves of this sort, such as measures against abrasive wear caused by contact with a high velocity fluid flow containing solid particles of minerals, measures against leakage of fluid caused by thermal strains of a valve body, valve stem or packing due to a temperature difference between an ambient temperature which is, for example, -60.degree. C. at the lowest and an internally running fluid temperature which is, for example, 150.degree. C. at the highest, packing methods of shutting off a maximum high pressure fluid of 2100 kg/cm.sup.2, methods of maintenance for facilitating replacements of worn-out parts, and so forth. Above all, it is a matter of utmost importance to provide a countermeasure against corrosion in consideration of properties of a fluid involved.
The fluid which occurs in a gas well or an oil well generally contains H.sub.2 S, CO.sub.2, chlorides, etc. so that it is acidic and very strong in terms of corrosiveness. In order to ensure corrosion resistance of valve components which contact a fluid of this sort under a high pressure, it has been the usual practice to select, for a major part such as a valve casing, a Cr--Mo--Ni steel which is a heat resistant steel with good castability and machinability. This kind of steel can be regarded as a corrosion resistant steel for its good corrosion resistance to a strongly oxidative acid such as nitric acid, but it is insufficient in corrosion resistance to non-oxidative acids. Accordingly, for securing a prolonged service life of a valve by improvement of its corrosion resistance along with enhancement of its resistance to abrasive wear and thermal shocks, it is necessary to provide laminated layers of high grade super alloys such as Inconel, Hastelloy, Waspalloy, Nimonic, Thetalloy, Udimet and the like on valve casing walls to be contacted with a corrosive fluid. However, integration of such high grade alloy materials into a valve casing as a whole inevitably results in high production cost and inferior cutting machinability.
The methods which are generally resorted to for the improvement of resistance to abrasive wear or corrosion of surfaces of a rigid steel structure include surfacing by welding, spraying, carburizing, nitriding, plating, explosive deposition, press-in fitting of the bushing and the like, which have respective merits and demerits. In this connection, cladding by forced press means such as rolling is also employed frequently, but it is not suitable for cladding the inner surfaces of a valve casing. Instead, a cladding method by an HIP (hot isostatic pressing) process has recently been introduced into the art (e.g., Japanese Patent Publication No. 56-156704), in which metal powder is packed into a gap spaced between a valve casing and an inner support structure and, after evacuating the inside, is put in a pressure container with a heater, sending as gas thereto from a superhigh pressure booster to produce moldings of metal powder under heated superhigh pressure conditions. This method still has a large number of problems to be solved, e.g., an increased number of steps in the manufacturing process and a high production cost, resulting from the use of the internal support structure and machining operation for removing the same subsequent to the HIP operation.